Catalytic oxidation unit for domestic oven exhaust



2 Sheets-Sheet 1 w. L. HENDERSON I CATALYTIC OXIDATION UNIT FOR DOMESTICOVEN EXHAUST Oct. 27,1970

Filed Feb WAYNE L. HENDERSON ms ATTORNEY Oct. 27 41970 w, HENDERSON3,536,?!-

CATALYTIC OXIDATION UNIT FOR DOMESTIC-OVENBXIIAUST Filed Feb. 21, 1968 2shms snm a was I F'IG.4

. l I I l I I I I I I I I I o "5 l 3o 4o so 6o INVENTOIL WAYNE L.neuoaasou HIS ATTORNEY United States Patent US. Cl. 23-288 2 ClaimsABSTRACT OF THE DISCLOSURE A catalytic oxidation unit useful ineliminating smoke and odors issuing from the exhaust vent of a domesticcooking oven, particularly a high temperature, selfcleaning oven using apyrolytic process for degrading food soils. The oxidation unit comprisesa hollow housing supporting a plurality of catalyst coated gas burnersof cellular ceramic construction and having a high surface-tovolumeratio. The housing has an intake port and an exhaust port and includes apartition which separates the housing into intake and afterburnercompartments; with the gas flow being directed from the oven cavity intothe intake compartment, then through the catalyst coated gas burnersinto the afterburner compartment of the housing, and finally venting tothe atmosphere through the exhaust port. Primary combustion air may besupplied to the intakecompartment, while secondary combustion air may besupplied to the afterburner compartment of the housing to createafterburning of the oven exhaust therein.

CROSS-REFERENCE TO RELATED APPLICATIONS The present invention is animproved modification of the ceramic oxidation unit described in thecopending ap plication of Bohdan Hurko and Raymond L. Dills, Ser. No.396,551, now Pat. No. 3,428,435, and a second copending application ofBohdan Hurko, Ser. No. 396,549, now Pat. No. 3,428,434, both of whichare assigned to the same assignee as is the present invention.

BACKGROUND OF THE INVENTION Considerable testing of ceramic oxidationunits in conjunction with the exhaust gases issuing from a self-cleaningbaking oven has indicated that there are several important factors toconsider in addition to the amount of surface area of the substrate onwhich the catalytic coating is supported. These additional factors arethe velocity of the hot exhaust gases passing over the catalyst and thelength of time that the gases are exposed to high temperatures withinthe oxidation unit before they are exhausted from the oxidation unit.Thus it is important to obtain complete burning of the gases so as toconvert all of the gases generated by the pyrolytic breakdown of thesoils on the oven liner to CO Two methods are used for automaticallycontrolling the velocity of the gases. First, the gas stream enteringthe oxidation unit housing is divided and directed in oppositedirections to pass through the ceramic burners, and then the flow isreversed as the combustion gases pass up and around a partition whichdivides the housing into two compartments. This second compartmentoperates at a high temperature to assist the completion of the oxidationprocess before the gases are returned to the kitchen atmosphere.Moreover, the length and size of each cell of the ceramic block willreduce the flow so that the hot gases remain in the oxidationtemperature longer.

The principal object of the present invention is to provide a ceramicoxidation unit with a gas flow pattern that automatically controls thevelocity of the hot gases and treats these gases at a high temperaturefor a suflicient amount of time while supplying these gases with ampleoxygen to insure complete degradation and conversion of the gases to COA further object of the present invention is to provide an oxidationunit with catalytic coated cellular ceramic blocks of the type describedand a delayed supply of primary air for these ceramic burners, as wellas a constant supply of secondary air downstream of the burners toinsure complete combustion.

SUMMARY OF THE INVENTION The present invention, in accordance with oneform thereof, relates to a corrugated ceramic, catalytic oxidation unitsuch as for use over the exhaust vent in the walls of an oven cookingcavity. This oxidation unit has walls forming a hollow housing with anintake port and an exhaust port, and there is a catalytic coated gasburner having a perforated ceramic substrate of deep cellularconstruction that is arranged at one or more sides of the intake port. Abafile is fastened over the top of the burner and also to certain sidewalls of the housing to form a first intake compartment upstream of theburners and a second afterburner or exhaust compartment downstream ofthe burner. These burners are adapted to be mounted so as to derive muchof their heat from the walls of the oven cooking cavity. Modificationsof this invention would include a delayed source of primary air upstreamof the ceramic burners and a continuous source of secondary airdownstream of the ceramic burners. Moreover, in order to increase theefficiency of the ceramic burners during the initial stage of itsoperation a supplementary heating means may be provided in the intakecompartment to increase the ambient temperature of the ceramic burner sothat catalytic action is started when the initial gases from the ovenreach the burner.

BRIEF DESCRIPTION OF THE DRAWINGS My invention will be better understoodfrom the following description taken in conjunction with theaccompanying drawings and its scope will be pointed out in the appendedclaims.

FIG. 1 is a left side, fragmentary, elevational view of a free-standingelectric range with a ceramic catalytic oxidation unit embodying thepresent invention furnished over the oven vent, there being some partsbroken away and others in cross-section to best show the invention andthe environment in which the invention is best suited to operate.

FIG. 2 is a perspective view on an enlarged scale of the ceramiccatalytic unit of the present invention with parts broken away toimprove the understanding of the interior construction of the unit withits subdivision into an intake compartment and an exhaust or afterburnercompartment, and also showing the primary air opening communicating withthe intake compartment and two secondary air openings communicating withthe exhaust compartment.

FIG. 3 is a cross-sectional elevational view of a modification of theoxidation unit of FIG. 2 showing a supplementary heater in the intakeport of the unit, as well as a thermal, time delay valve normallyclosing the primary air opening in the lower temperature range of theintake compartment.

FIG. 4 is a temperature-time chart comparing the change of thetemperatures of the two ceramic burners with respect to the change oftemperature of the air at the center of the oven cavity.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to a considerationof the drawings, and in particular FIG. 1, there is shown forillustrative purposes an electric range 10 having a top cooking surfaceor cooktop 11 with a plurality of surface heating elements 12, an ovencavity 13 located beneath the cooktop and formed by a box-like ovenliner 14 and a front-opening, drop door 15. The oven cavity is suppliedwith two standard heating elements; namely, a lower baking element 16and an upper broiling element 17 that may be furnished with an invertedreflector pan 18 which overlies the same for directing radiant energy ina downward direction during broiling operations. The surface heatingelements 12 are controlled by selector switches 20 located in thesidearms of the cooktop 11, while the oven heating elements 16 and 17are controlled by a combined oven selector switch and oven thermostat 21which is positioned in the control panel 22 of a backsplasher 23 that isvertically positioned along the back edge of the cooktop 11.

As is conventional in this art, the oven liner 14 is surrounded by athick layer of thermal insulating material 25, such as fiberglass or thelike, for retaining the heat generated within the oven cavity. A rangebody or cabinet structure 26 forms the external structure of the rangefor supporting the various elements therein, and it is provided with anouter appearance finish of porcelain enamel, stainless steel or the likeas is conventional in this art.

It is standard practice to provide an oven with a vent or exhaust ductcommunicating with the kitchen atmosphere; especially, for use duringbroiling operations when a large volume of room air is passed throughthe oven, usually by means of a partially open oven door so as to holddown the oven air temperature. The broiling operation utilizes a maximumtemperature setting of the oven thermostat to obtain a steady ONcondition of the broil element 17. Moreover, it is necessary in a hightemperature, self-cleaning oven which operates on a pyrolytic principleto provide an oxidation unit such as 28 in the oven vent system todegrade the oven gases and remove the smoke and odors before the gasesare returned to the kitchen atmosphere. In place of a standard exhaustduct I have attached on the top wall of the oven liner a catalyticoxidation unit 28 comprising the present invention with walls forming ahollow housing 29 that is positioned over the oven vent opening 31 inthe top wall of the oven liner 14.

The particular nature of the oxidation unit 28 can best be understood bystudying the perspective view of FIG. 2 Which is taken from the lefthand, rear corner of the range 10 looking down upon the oxidation unit.First to help in the orientation, there is the top wall of the ovenliner 14 with the oven vent opening 31 cut therein. The hollow housing29 is formed by a front wall 33, a back wall 34, opposite side walls 35and 36 and a top wall 37. Notice that there is no bottom wall, and thatthe vertical walls 33-36 are each provided with an outwardly turnedflange 39 on the lower edge thereof so that fastening screws 41 may beinserted through suitable holes therein and through holes in the ovenliner to fasten the housing 29 to the top wall of the oven liner.

The oven exhaust vent opening 31 constitutes the intake port for theunit 28. Arranged on opposite sides of this port is a pair of catalyticcoated, perforated ceramic blocks 43 and 44 of thin wall cellularconstruction. One suitable source of this ceramic material is theMinnesota Mining and Manufacturing Company through its American LavaCorporation. This material is designated by them as corrugated ceramic.Another available source of this ceramic material is the E. I. Du Pontde Nemours Company, which designates this material as honeycomb ceramic.Two types of ceramic that have been found successful are cordierite andalumina. The term corrugated ceramic is apparently derived from the factthat the material is of cellular construction having a configuration ofmultple layers of corrugated paper in that there is one series ofspaced, flat partitions where each pair is separated by a corrugatedspacer. The corrugations are of a size with about seven corrugations perinch, and the width of each cell is about .100 inch. The depth of theblock is about one inch. The length and size of the cells of the burnerare also designed to reduce the flow so that the hot gases remain at theoxidation temperature for a longer period of time. Thus it can be seenthat the ceramic block is indeed perforated and of cellularconstruction. The designation honeycomb ceramic apparently comes fromthe fact that the cells are cut in transverse cross-section, therebygiving it the appearance of cells of honey built by honeybees.

A catalytic material such as platinum or other precious metal is appliedas a thin coating to the ceramic blocks 43 and 44, and including theinterior surfaces of the cells thereof which are to come into contactwith the hot oven gases flowing therethrough.

The normal cooking temperatures in a domestic oven vary between about F.and about 550 F. Within recent times a self-cleaning oven design hasbeen introduced on the market and manufactured according to theteachings of the Bohdan Hurko Pat. 3,121,158 that was mentioned earlier.Such an oven design uses a pyrolytic process for removing the food soilsand grease splatter that accumulate on the surfaces of the oven linerand inner door panel. This is done by raising the oven temperatures to aheat cleaning temperature somewhere above about 750 F. and holding itfor a suflicient length of time to reduce the food soil and greasesplatter into gaseous products which are further degraded in a catalyticoxidation unit such as unit 28, before the gases are returned to thekitchen atmosphere.

One of the first catalytic oxidation units used in the commercialself-cleaning oven was built according to the teachings of the patent ofStanley B. Welch 2,900,483, which is likewise assigned to the assigneeof the present invention. Welch discloses a spiral, metal sheathedresistance heating element used in conjunction with a platinum coatedwire screen that is interleaved in the turns of the spiral for oxidizingthe smoke and odors and grease that might be present in the oven exhaustgases. This Welch design is a most satisfactory design from anoperational viewpoint, yet the use of a metal sheathed heatercontributes a great deal to its unit cost. Moreover, the woven wiresforming the screen have a very small surface area, and they arecontacted by the gases for only a fleeting moment as compared with theaction of elongated cells of the ceramic blocks of the presentinvention.

The heating of the food soils and grease splatter within the oven cavityduring the heat cleaning operation produces corresponding primarygaseous degradation products which are combined with a controlled amountof ambient air drawn into the oven cavity usually through a small gaparound the oven door. Such degradation includes methane, ethane, watervapor, carbon monoxide, some free carbon and other elements. Smoke,odors and other underisable products are initially generated attemperatures of around 300 F., and it is important to be able toeliminate these before the hot oven gases are returned to the kitchenatmosphere.

Considerable testing has indicated that the amount of carbon monoxide inthe exhaust stream coming from the oxidation unit 28 is related to theamount of odor being produced. This is an observation that has been madeduring a number of tests in which carbon monoxide was being measured.Carbon monoxide is, of course, odorless, but it signals a condition ofincomplete burning and a product which has odor must be present. Testshave also shown that the velocity of the gases being passed through theoxidation unit effect its efliciency. The present invention willautomatically control the velocity of the gases and thus will producemore complete burning and convert the oven gases to carbon dixide whichis odorless. The oxidation unit 28 has an open bottom wall that ismounted directly to the top wall of the oven liner 14 so that theceramic blocks 43 and 44 are bearing directly on the surface of the ovenliner. Thus the ceramic blocks are heated by the oven liner and thisreduces the temperature diiferential between the oven exhaust gastemperature and the temperature of the ceramic blocks. In the vicinityof the air intake opening 31 turbulence is produced and this in turnreduces the gas velocity so that the gas is in contact with thecatalytic surfaces of the ceramic blocks 43 and 44 for a longer periodof time than if the gases pass directly through the ceramic blockswithout turbulence.

Looking at FIGS. 2 and 3, there is a supplementary air chamber 47 insidethe back wall 34, and it is defined by a vertical, parallel wall 49,opposite side walls 35 and 36, the top wall of the oven liner 14 and acover plate 51 which covers both the air chamber 47 and the two ceramicburners 43 and 44. Thus, the cover plate 51 creates an intakecompartment 53 over the oven vent opening 31 and between the two burners43 and 44.

The portion of the cover plate 51 over the two burners stops short ofthe side walls 35 and 36 and actually terminates at the rear edge ofeach burner as is best seen in FIG. 2. Thus, the gases divided and flowthrough one of the burners and the side wall deflects the flow upwardinto an afteburner or exhaust compartment 55 where the gas flows backover the top of the cover plate 51 and passes out of the exhaust port57.

There is a primary air inlet 60 in the center of the wall 49 of thesupplementary air chamber 47 and a pair of secondary air inlets 61 and62, one at each end of the wall 49 to be located downstream of theburners 43 and 44. The back wall 34 is furnished with a largerectangular air inlet 64, as is best seen in FIG. 2. Over this inlet 64is fastened a thin but wide air duct 66 that extends rearwardly to theback wall of the range and is open to the atmosphere. The supplementaryair input system for primary and secondary air is so constructed thatthis air is raised in temperature above room ambient temperature priorto entering the combustion area of the oxidation unit. This heating ofthe supplementary air supply, as it passes through the duct 66 that isburied in the thermal insulation 25, improves the efficiency of theoxidation cycle.

Notice that the primary air inlet 60 is provided with a bimetal valve orshutter 68 of cantilever construction which is supported from the wall49 and normally closes the inlet 60 at gas temperatures below about 750F. Above this temperature or other more suitable temperature, thebimetal 68 is open to supply primary air to the intake compartment 53.Moreover, the secondary air inlets 61 and 62 are always open to theatmosphere. The size of the secondary air inlets 61 and 62 automaticallycontrols the amount of air supplied because the change in temperaturebetween ambient air and the exit of the burners 43 or 44 is a functionof the amount of soil being oxidized.

In the second modification of FIG. 3, an auxiliary resistance heater 70of wire or screen formation is assembled across the intake port 31 ofthe oxidation unit 28 to raise the ambient temperature of the gasesentering the intake compartment 53 especially during the beginning ofthe cycle as was mentioned heretofore. The heater 70 is supported from aring 72 of insulating material that is in turn fastened over the intakeport 31 by screw fasteners 74. The heater is adapted to be electricallyconnected to a 120 volt circuit for energizing the heater, when desired.

Looking at the time-temperature graph of FIG. 4, the lower curve Arepresents the variation of the oven air temperature with time, whilethere are two upper curves labeled 43 and 44 as they represent thesurface temperatures of the two ceramic burners 43 and 44 respectively.

Modifications of this invention will occur to those skilled in this art.Therefore, it is to be understood that this invention is not limited tothe particular embodiments disclosed, but that it is intended to coverall modifications which are within the true spirit and scope of thisinvention as claimed.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A smoke and odor oxidation unit that is adapted to be mounted overthe exhaust vent in the walls of an oven cooking cavity, the unit havingwalls forming a hollow housing with an intake port and an exhaust port,and a plurality of catalyst-coated gas burner means with a perforatedceramic substrate of deep cellular construction and a high ratio ofsurface area to volume, at least two of such burner means being arrangedwithin said hollow housing and located on at least two of the oppositesides of the intake port, bafile means located in said housing andarranged over the tops of said two burner means and joined to certainside walls of said housing so as to partition the hollow housing into afirst intake compartment located upstream of said two burners with saidintake port communicating therewith and a second afterburner compartmentlocated downstream of said two burners with said exhaust portcommunicating therewith wherein the gas flowing into said intakecompartment is divided into a plurality of streams which flow throughsaid at least two burner means into the afterburner compartment, saidburner means being adapted to derive much of its heat by conduction fromthe walls of the oven cooking cavity, and electrical resistance heatermeans located adjacent the first intake compartment for use at leastduring an initial preheat stage to heat up the gases entering thehousing from the cooking cavity, the walls of the hollow housing havinga primary'air inlet opening communicating with the intake compartmentand a secondary air inlet opening downstream of each of said two burnermeans in the afterburner compartment, and a normally closed thermallyresponsive valve cooperating with the primary air inlet opening andoperable to open at temperatures of about 750 F. and above to allowprimary air to mix with the hot gases issuing from the oven cookingcavity vent before passing through the burner means.

2. A smoke and odor oxidation unit as recited in claim 1 wherein asingle air duct communicates with both the primary air inlet opening andthe two secondary air inlet openings, the said air duct being adapted tobe heated by its surrounding environment so as to preheat both theprimary and secondary air before they enter the oxidation unit.

References Cited UNITED STATES PATENTS 1,757,987 5/ 1930 Whittier.2,845,882 8/ 1958 Bratton. 2,953,357 9/1960 Long. 3,110,300 11/1963Brown et al. 3,166,895 1/1965 Slayter et al. 3,220,179 11/ 1965Bloomfield. 3,325,256 6/ 1967 Calvert. 3,428,435 2/1969 Hurko et al.3,470,354 9/1969 Tilus 12621 XR MORRIS O. WOLK, Primary Examiner B. S.RICHMAN, Assistant Examiner U.S. Cl. X.R. 1262l; 219396

